Process for making monocrystalline silicon

ABSTRACT

DISCLOSED IS A PROCESS FOR MAKING MONOCRYSTALLINE SILICON IN A ZONE REFINING OPERATION. IN THIS PROCESS, A GASEOUS MIXTURE COMPRISING PREDETERMINED VOLUME PERCENTAGES OF HYDROGEN GAS AND A SELECTED INERT GAS, SUCH AS ARGON, IS INTRODUCED OR &#34;PURGED&#34; INTO A CHAMBER OF A ZONE REFINER BOTH PRIOR TO AND DURING A SILICON ZONE REFINING OPERATION. THE PRESENCE OF A SELECTED VOLUME PERCENTAGES OF NO MORE THAN 5% HYDROGEN GAS IN THIS GASEOUS MIXTURE WHICH IS PURGED INTO THE ZONE REFINING CHAMBER HAS BEEN FOUND TTO ELIMINATE SWIRLS IN THE SILICON WHICH HAD PREVIOUSLY BEEN DISCOVERED AFTER ETCHING ZONE REFINED SILICON WAFERS PROCESSED IN A ZONE REFINING OPERATION USING NO HYDROGEN GAS.

PROCESS FOR MAKING MONOCRYSTALLINE SILICON Fi led Aug 13 1970 FIG! INERT GAS .t

FIG. 2 @M IQ d 70 ,Y A l 72 l I l INVENTOR K l Y I WLLas H. CAMPaLL V 70 BY @Si www ,Qwzum ATTORN EY United States Patent Office 3,734,695 Patented May 22, 1973 U.S. Cl. 23-301 SP 10 Claims ABSTRACT OF THE DISCLOSURE Disclosed is a process for making monocrystalline silicon in a zone refining operation. In this process, a gaseous mixture comprising predetermined volume percentages of hydrogen gas and a selected inert gas, such as argon, is introduced or purged into a chamber of a zone refiner both prior to and during a silicon zone refining operation. The presence of a selected volume percentage of no more than 5% hydrogen gas in this gaseous mixture which is purged into the zone refining chamber has been found to eliminate swirls in the silicon which had previously been discovered after etching zone refined silicon wafers processed in a zone refining operation using no hydrogen gas.

FIELD OF THE vINVENTION This invention relates generally to a float zone process for forming monocrystalline silicon and more particularly to the production of such silicon which is relatively free of swirls.

BACKGROUND OF THE INVENTION The oat zone or zone melting process for fabricating monocrystalline silicon has been well known for several years. In this process or variations thereof, a molten zone is initially produced at a selected location on a silicon rod by inductive radio frequency (RF) heating. Frequently, auxiliary heating means, such as a conductive heating plate, lis utilized initially in combination with the RF inductive heating to rapidly heat up the silicon rod and reduce the zone refining start-up time. Thereafter, the molten zone is moved along the silicon rod by the production of relative motion between the silicon rod and an adjacent RF induction heating coil. Such relative motion may be provided by well known mechanical means, such as motor driven shafts, and the traverse rate of the molten zone along the silicon rod will depend upon the size of the molten zone employed and upon the power of the RF field produced by the adjacent induction heating coil. In the selection of the above parameters to determine this traverse rate, care must be taken to prevent zone fallout as will be appreciated by those skilled in the art.

DESCRIPTION OF THE PRIOR ART Hitherto, it has been a standard procedure in the zone refining of elemental silicon to either: (1) carry out the zone refining operation in a vacuum which has been pulled on the chamber surrounding the silicon rod, or (2) use an inert gas, such as argon, to displace the air or other gas in the zone refining chamber prior to beginning the zone refining operation. The reason that the above procedures are employed is that molten silicon adversely reacts with oxygen in the zone refining chamber, with the resultant zone refined silicon rod having an undesirably high oxygen content therein.

The maintenance of a high vacuum condition within a zone refining chamber has several attendant disadvantages, among which include the requirement for bulky and expensive vacuum equipment which must be employed with each zone refiner. Additionally, vacuum systems can never completely remove all oxygen from the zone refining chamber. For these reasons, inert gases, such as argon, have frequently been used to replace vacuum systems and thus enable the zone refining operation to be carried out in an inert gaseous atmosphere which is relatively free of oxygen.

Monocrystalline silicon wafers which have been made utilizing the above inert gas zone refining prior art method have exhibited swirls which consist of clusters of imperfections that appear as shallow annular pits or hillocks on the etched planar surface of the wafer. The swirls were discovered, for example, after some of the wafers had been etched with a selected sirtl etchant which is described below in detail. These swirls tend to lower the minority carrier lifetimes in the monocrystalline silicon and further act as preferential diffusion paths for irnpurities during the zone refining operation.

SUMMARY OF THE INVENTION The general purpose of this invention is to provide new and improved high quality swirl-free monocrystalline silicon and a zone refining process for making same. The present inventive process includes all of the advantages of present state-of-the-art processes for producing monocrystalline silicon, and yet produces a monocrystalline silicon which is free of these previously discovered swirls on the etched surface of the silicon wafers. To attain this, a chosen amount of hydrogen gas is introduced or purged into the zone refining chamber along with an inert gas both in preparation for and during a zone refining operation. It has been observed that by so purging the zone refining chamber with a controlled mixture of a hydrogen gas and a selected inert gas, the swirls in the monocrystalline silicon can be eliminated.

Accordingly, an object of the present invention is to provide an improved zone refined monocrystalline silicon which is free of swirls.

Another object of this invention is to provide a new and improved zone refining process for the production of monocrystalline silicon. The swirl-free monocrystalline silicon produced by the present process exhibits a desirable high minority carrier lifetime characteristic.

These and other objects and features of this invention will become more fully apparent in the following description of the accompanying drawing.

DRAWING FIG. 1 is a perspective view of a silicon zone refining chamber and associated purge gas flow lines which may be utilized in performing the process according to the present invention. The zone refining apparatus shown in FIG. 1 is in its pre-startup condition.

FIG. 2 shows the same zone refining apparatus illustrated in FIG. 1 and in operation during a zone refining pass in which the present inventive process is carried out.

THE INVENTION Referring to FIGS. 1 and 2 of the drawing, the reference numeral 10 represents a support panel for a zone refining chamber, generally designated by the reference numeral 12. The chamber 12 comprises a back wall 14, sidewalls 16 and 18, a top wall 20 and a bottom wall 22. In FIGS. 1 and 2, the front wall of the chamber 12 opposite the back wall 14 has been -omitted so that the zone refining apparatus in the chamber 12 can be seen and described.

An elongated polycrystalline silicon rod 26 is secured to the top `wall 20 of the chamber 12 by a rod holder 24 which can be of conventional construction. The rod holder 24 is designed to hold the silicon rod 26 such that it extends substantially in the vertical direction in the chamber 12. While the position of the silicon rod 26 can deviate from a strictly vertical position, any substantial deviation is generally undesirable since it promotes the falling out of the molten zone of silicon and may necessitate the use of levitation means to prevent the loss of the molten zone.

yBeneath the lower end of the polycrystalline silicon rod 26 is a monocrystalline silicon seed crystal 32 which is held in the vertical position shown by a seed holder chuck 30. The seed holder chuck 30 is supported atop a rotatable shaft 28 which extends through the bottom wall 22 of the zone refining chamber 12. The rotatable shaft 28 is longitudinally movable so that the silicon seed crystal 32 can be fused to the lower end of the silicon rod 26 as shown in FIG. 2 during the initial phase of the zone refining operation.

Surrounding the silicon rod 26 is an RF heater coil 34 joined to conductive arms 36 and 38 which are securely attached to a transverse member 40 on the traverse rod 41. The traverse rod 41 may be driven by any suitable means (not shown) to rotate the rod 41 and thus move the transverse member 40 up the silicon rod 26 during the zone refining operation. However, it should be understood that the present invention is not limited to this type of uprelining, nor is it limited to any mechanical means for producing the relative movement between the R-F work coil 34 and the silicon rod 26.

The above-described zone refining apparatus Within the chamber 12 is accessible to an operator via a front panel 44 having a door handle 42 thereon as shown. iFor a more detailed discussion of the type of the zone refining apparatus shown in the drawing, reference may be made to U.S. Pat. 3,251,658, issued to A. Hambach et al. on May 17, 1966 and assigned to the present assignee.

A pair of gas tiow lines 46 and 48 are connectable to the indicated sources (not shown) of hydrogen gas and a selected inert gas, respectively. The hydrogen gas, H2, and the inert gas are controlled by gas flow valves 50 and 54 in a manner to be described hereinafter, and the. regulated hydrogen and inert gases are passed through gas lines S6 and 58 to a common flow control valve 60 which regulates the total volume of the H2-inert gas mixture entering the zone refining chamber 12 via gas line 62 and opening 64. The total gaseous mixture introduced or purged via opening 64 may be controlled in accordance with the opening or setting of the valve 62 and is also dependent upon the gas pressures within the individual gas lines 46 and 48. An exit gas line 68 having an opening 66 therein extends to the outside of the zone refining chamber 12, and the gaseous mixture exiting the chamber 12 is controlled by another suitable fiow control valve 70. The flow or purge control of inert gases introduced into a silicon zone refining operation is well known in the semiconductor art and is therefore not described in detail herein.

ZONE REFINING STARTUP AND OPERATION Prior to beginning the first zone refining pass, and With the zone refining apparatus in the position shown in FIG. 1, a mixture of hydrogen gas, H2, and a selected inert gas is passed through the flow control valve and into the zone refining chamber 12 to displace the air or other gases in the chamber 12. This gaseous mixture enters the chamber 12 via line 62 and exits the chamber 12 via line 68 at a continuous purge or liow rate. This gas purging of the chamber 12 is allowed to continue for approximately 20 minutes prior to beginning the Zone refining operation to be further described.

A preferred range by volume percentages of hydrogen gas to the total gaseous mixture entering the chamber 12 is from between approximately 0.1 and 5.0%, and a corresponding preferred range of the selected inert gas entering the chamber 12 is from between approximately 99.9% and respectively. Within this preferred range, a gaseous mixture of approximately 1% H2 and 99% inert argon gas has been used commercially to eliminate all swirls in the zone refined silicon. These gas volume percentages have been successfully used continuously during both pre-startup gas purging and the continuous dynamic purging of the zone refining chamber during a zone refining operation. However, by appropriately controlling the flo-w control valves 50 and 54, these volume percentages may be varied within the above range to produce swirl-free monocrystalline silicon within the scope of the present invention.

It has been observed that when more than approximately 2% H2 in the gaseous purging mixture is used, a subsequent undesirable preferential etching of the zone rened silicon will occur when the zone refined silicon water are etched in CP4. This etchant is a mixture of acetic acid, nitric acid and hydrofluoric acid which is sold commercially under the trade name CP4. The latter etching step is utilized prior to the sirtl etch step referred to above, and the acid etchant CP4 isI applied to wafers obtained from the zone refined monocrystalline silicon rods to remove the saw damage therefrom. When slightly more than 2% H2 gas is used in the purging mixture enteringthe chamber 12, the etchant CP4 has been found to preferentially etch the monocrystalline silicon wafers and thereby leave dished-out areas on the etched wafer surface. This latter undesirable preferential etching has been found to occur only on wafers that were sliced very near the seed end of the monocrystalline silicon rod. If the percentage of H2 gas in the gaseous purging mixture is increased to approximately 5%, this undesirable preferential etching of the silicon wafers by CP., becomes noticeably increased and substantially reduces Wafer processing yields.

The inert gas entering line 48 may be selected from the group of inert gases consisting of helium, argon and krypton. This inert gas is purged in a gaseous mixture with the hydrogen gas in the above-described percentages via the gas line 62 into the zone refining chamber 12, both prior to and during the zone refining operation. Advantageously, the pressure within the zone refining charnber 12 is maintained between 0.8 and 2.0 atmospheres, and this pressure may be regulated in accordance with the pressures of the sources of hydrogen and inert gas and the valve settings of the gas intake iiow control valves 50, 54 and 60 and the exit flow control valve 70.

After the zone refining chamber 12 of FIG. 1 has been purged with the above mixture of hydrogen and a selected inert gas for approximately 20 minutes, the silicon zone refining operation is continued by fusing the monocrystalline seed crystal 32 to the bottom of the polycrystalline silicon rod 26 at a selected elevated zone refining temperature. This temperature is typically in the vicinity of 1420 C. Once the seed crystal 32 is fused to the silicon rod 26, a liquid molten zone will begin to form on the bottom of the silicon rod 26, and this zone may be moved or passed up the silicon rod 26y as the RF induction heating coil 34 is moved vertically up the traverse rod 41. This liquid molten zone 35 is shown in FIG. 2 and is slightly smaller in diameter than the diameter of the polycrystalline silicon rod 26. As the molten zone 35 moves up the silicon rod 26, the upper interface between the zone 3S and the rod 26 is continuously melting whereas the lower interface between the Zone 35 and the silicon rod 26 resolidifles. The molten zone 35 may be moved along the silicon rod 26 one or more times, if desired, and each of these movements is referred to as a single passi,

The gaseous mixture of hydrogen and an inert gas is purged into the chamber 12 throughout the complete zone refining operation. Using 1.0% H2 gas and 99.0% argon in the purging mixture, the H2-argon mixture continuously purged into the opening 64 in the gas line 62 during a zone refining operation. By purging the zone refining chamber 12 with the above H2-argon gaseous mixture, monocrystalline silicon rods with orientations in the 111, and 110 planes have been formed from polycrystalline rods and have ranged in resistivity from 0.1 ohm centimeter to intrinsic. Both P and N type swirl-free monocrystalline silicon rods have been made using the abovedescribed process.

Wafers sliced from these monocrystalline silicon rods and previously etched with CP4 as noted above, are further etched in a selected sirtl etchant. This sirtl etchant is used to expose so-called dislocations on the wafer surface and will also expose the swirls on the wafer surface if any exist. Wafers which were fabricated using the abovedescribed gas purging process according to the present invention and then etched with a selected sirtl etchant were found to be completely free of any swirls on the etched planar surface thereof. The particular sirtl etchant -used was a solution of distilled water, chromium trioxide (CIOs) and hydrofiuorine acid (HR). In preparing this solution, 100 grams of 99.9% CrO3 and 100 grams of 48- 51% HF were added to 200 milliliters of distilled water.

The invention described above is not limited to the particular zone refining apparatus shown in FIGS. 1 and 2, and any other` suitable zone refining apparatus and appropriate gas flow control apparatus equivalent to that shown in the drawing may be utilized within the scope of this invention.

I claim:

1. A process for fabricating swirl-free monocrystalline silicon in a chamber of a Zone rener which includes introducing predetermined volume percentages of hydrogen gas from approximately between 0.1% and by volume and a selected inert gas from between approximately 99.9% and '95% by volume, respectively, into said chamber to displace existing gases therein during a zone rening operation and thereby eliminate swirls from silicon prepared in said zone rener.

2. The process defined in claim 1 wherein said hydrogen gas is introduced into said chamber at approximately 1.0% by volume and said selected inert gas is introduced into said chamber at approximately 99.0% by volume.

3. The process defined in claim 1 wherein said inert gas is selected from the group of inert gases consisting of argon, helium and krypton.

4. The process defined in claim 3 wherein said hydrogen gas is introduced into said chamber at approximately 1.0% by volume and said select inert gas is introduced into said chamber at approximately 99.0% by volume.

5. A process for fabricating swirl-free monocrystalline silicon in a zone refining operation which includes the steps of z (a) displacing the air within a zone refining chamber by purging said chamber with a mixture of hydrogen gas from approximately between 0.1% and 5% by volume and a selected inert gas from approximately between 99.9% and 95% by Volume, respectively, for a predetermined time prior to beginning a zone refining operation, and

(b) making a zone refining pass on a polycrystalline silicon rod to convert said rod to monocrystalline silicon while simultaneously purging said zone relining chamber with the above said mixture of hydrogen gas and a selected inert gas at a predetermined flow rate through said chamber during said zone refining operation.

6. The process defined in claim S wherein said mixture of hydrogen gas and said selected inert gas is purged into said zone refining chamber for a time interval prior to starting the zone refining operation sufficient to displace substantially all existing gases within said chamber and thereby reduce the oxygen content therein prior to initiating a Zone refining operation.

7. The process defined in claim 6 wherein said hydrogen gas is introduced into said chamber at approximately 1.0% by volume and said selected inert gas is introduced into said chamber at approximately 99.0% by volume.

8. The process defined in claim 6 wherein said selected inert gas is selected from the group of gases consisting of argon, helium and krypton.

9. The process defined in claim 7 wherein said inert gas is selected from the group of gases consisting of argon, helium and krypton.

10. In a process for fabricating monocrystalline silicon in a zone refiner which includes purging a chamber of the zone refiner with a gaseous mixture including an inert gas to displace oxygen within the chamber both prior to and during a silicon zone refining operation, the improvement comprising: introducing a selected volume percentage of hydrogen gas from between approximately 1.0% and 2.0% by volume into said purging mixture to eliminate any swirls in the monocrystalline silicon prepared in the Zone refiner.

References Cited UNITED STATES PATENTS 3,069,244 12/1962 Sterling 23-301 3,086,850 4/1963 Marino et al 23-301 3,206,286 9/1965 Bennett et al. 23-301 3,172,734 3/1965 Warren 23-301 3,265,469 8/1966 Hall 23-273 3,273,969 9/1966 Sirgo 23-273 3,314,769 4/1967 Rudness 23-301 3,362,795 1/1968 Weisbeck 23-294 3,416,898 12/1968 Shiroki et al 23-301 3,530,011 9/1970 Suzuki et al 23-301 3,660,062 5/1972 Keller 23-301 FOREIGN PATENTS 839,944 4/1970 Canada 23--301 1,023,023 1/1958 Germany 23-301 OTHER REFERENCES Hannay: Semiconductors, 1959, Reinhold Pub. Co., New York, pp. 527-5 30.

NORMAN YUDKOFF, Primary Examiner R. T. FOSTER, Assistant Examiner U.S. Cl. X.R. 23--294 

